This report presents information collated on the earth pressures and settlements that
develop behind model and full-scale integral bridge abutments. The objective is to
facilitate the design of integral bridges; for which the current UK guidelines are
arguably overly conservative. The report concludes that integral bridge design lengths
should be incrementally increased. Modifications to BA 42/96 are suggested based on
measured earth pressure increases due to cyclic loading. With adequate compaction
and drainage, approach slabs are unnecessary.

INTRODUCTION

An integral bridge may be defined as having no expansion joints or sliding bearings,
the deck is continuous across the length of the bridge. Integral bridges are
alternatively referred to as integral abutment bridges, jointless bridges, integral bent
bridges and rigid-frame bridges. Semi-integral or integral backwall bridges typically
have sliding bearings, but no expansion joints.
Expansion joints and bearings have traditionally been used to accommodate the
seasonal thermal expansion and contraction of bridge decks, typically of the order of
tens of millimetres. A survey of approximately 200 concrete highway bridges in the
UK, carried out for the Department of Transport, however, revealed that expansion
joints are a serious source of costly and disruptive maintenance work (Wallbank, 1989
cited in Springman et al., 1996). In response to this, the Highways Agency published
Advice Note BA 42 in 1996, promoting the design of integral bridges and stating that
all bridges up to 60m in length should be integral with their supports.
Although the integral bridge concept has proven to be economical in initial
construction for a wide range of span lengths, as well as technically successful in
eliminating expansion joint/bearing problems, it is susceptible to different problems
that turn out to be geotechnical in nature. These are potentially due to a complex soilstructure
interaction mechanism involving relative movement between the bridge
abutments and adjacent retained soil. Because this movement is the result of natural,
seasonal thermal variations, it is inherent in all integral bridges.
There are two important consequences of this movement:
1) Seasonal and daily cycles of expansion and contraction of the bridge deck can lead
to an increase in earth pressure behind the abutment. This build-up of lateral earth
pressures is referred to as 'soil ratcheting' (England & Dunstan, 1994 cited in England
et al., 2000). This can result in the horizontal resultant earth force on each abutment
being significantly greater than that for which an abutment would typically be
designed and represents a potentially serious long-term source of integral bridge
problems.
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2) The second important consequence is the soil deformation adjacent to each
abutment. It has been postulated that settlement troughs occur as a result of the soil
slumping downward and toward the back of each abutment. In many cases this is
addressed by the incorporation of an approach slab into the bridge design, whereby
the slab is intended to span the void created underneath it. However, there is also
evidence to suggest that such a slab is unnecessary and that regular maintenance of
the bridge surface can be sufficient to largely overcome this problem.
1.1 Purpose and Scope of Project
Guidance on the design of integral bridges in the UK can be found in BA 42/96;
aspects of this document, however, are regarded as being overly conservative
(England et al. 2000). These issues are currently being addressed by the Highways
Agency and the Advice Note is in the process of being updated. New variations of
integral bridge designs are continuously emerging, however, and it is important that
design guidelines are not inappropriately used in such cases. The process of
modifying this document is therefore inherently time-consuming.
In the interim, some bridge designers are reaching agreements on departures from the
code with the Highways Agency, based on more recent research findings. The aim of
this report is to draw information from a wide variety of sources in order to increase
the confidence of designers in the performance of integral bridges and subsequently
facilitate this design process.
Numerical modelling will undoubtedly become an increasingly powerful tool for the
design and analysis of integral bridges, but time constraints on a project of this type
have resulted in the focus being placed solely on experimental testing and field
testing. It is hoped that these results may also be used to help improve numerical
modelling techniques. This report is also limited to quantifying the earth pressures
and settlement behind an abutment, rather than postulating a mechanism for this
behaviour.
Integral bridges are not a new concept. The first section of the M1, constructed in
1958-59, required 127 bridges, of which 88 are of a continuous portal type, that act
integrally with the surrounding soil and range in span up 41m. Various integral bridge
construction techniques are widely and successfully used in the USA, Sweden,

Canada and Australia, which include spans up to 100m (Burke, 1989; Hambly 1991 -
cited in Springman et al., 1996). The boundaries of design are being pushed further
still with the use of innovative backfill materials, leading to a bridge in the USA
totalling 300m in length, which is also performing well. (Frosch, 2002).

Mode of Bridge Movement

With the elimination of expansion joints, the thermal expansion and contraction of the
bridge deck must be alternatively accommodated. Card & Carder (1993) postulated
that for portal frame bridges, such as those found on the M1, this could be achieved
by vertical deflection of the bridge deck, rather than by longitudinal thermal
movements of the bridge deck being transmitted to the abutments. Subsequent
experimental research on two such bridges by Darley and Alderman (1995), however,
concluded that vertical movements were generally very small, effectively disproving
this theory. These findings are supported by results from field studies on a shallow
abutment bridge supported by piles (Lawver, 2000) and a shallow spread-base
abutment bridge (Darley et al., 1998) which showed that the primary abutment
movement was horizontal translation. Embedded abutments that are pinned at their
base, however, rotate about the toe of the abutment wall (Barker & Carder, 2001).

Magnitude of Deck Expansion

The magnitude of the longitudinal deck expansion is dependent primarily on the
bridge temperature. Extensive research carried out in the UK (Emerson, 1973, 1976,
1977, cited in England et al., 2000) has resulted in temperature parameter, the
effective bridge temperature δTEB (or EBT), which relates well to the shade
temperature. The research resulted in published EBT values for concrete, composite
(steel-concrete) and steel box section bridges in different geographical locations in the
UK (Emerson, 1976). Composite and steel decks may be assumed to have the same
coefficient of thermal expansion as concrete, but they experience higher changes in
EBT, so that the seasonal movements of composite decks and steel decks are about
121% and 145% of that of a concrete deck, respectively (England et al., 2000).
This movement will be combined with deck strains (since the deck axial load must be
in equilibrium with the lateral resistance of the soil behind the abutment) and post
construction effects such as shrinkage.

Bridges in single words can be defined as bridges without joints. The integral bridges are characterized by a monolithic connection between the roof and the substructure (pillars and pillars). They span from one pillar, on intermediate support to the other pillar, without any gasket on the cover. Comprehensive bridges have been built all over the world, including India.

Bridges built without any expansion joints (between spans or between spans and pillars) and without any bearing are called integral bridges.

CHARACTERISTICS OF INTEGRAL BRIDGES

The concept of integral pillar bridge is based on the theory that due to the flexibility of the stack, the thermal stresses are transferred to the substructure by a rigid connection between the superstructure and the substructure.

MOTIVATION OF INTEGRAL BRIDGES

• To remove the expansion joints on the cover
• When seismic forces are predominant or when considering increased explosion resistance, the concept of integral bridge is an excellent choice.
• Less expensive
• Increased durability
• Easy to design